As we enter the multimedia age, there are demands to increase the capacity and speed of data processing in optical communication systems and computers, and transmission systems that use light as a transmission medium have come to be used in LANs (local area networks), FA (factory automation), interconnection between computers, household wiring and so on.
Optical waveguides used in such transmission systems are fundamental constituent elements in, for example, optical devices, optoelectronic integrated circuits (OEICs) and optical integrated circuits (optical ICs) for realizing high-capacity data transmission of movies, moving pictures and so on, and for realizing optical computers and so on. Due to mass demand, assiduous research is being carried out into optical waveguides, and in particular, high-performance and low-cost products are demanded.
Until now, quartz optical waveguides and polymer optical waveguides have been known. Of these, quartz optical waveguides have better transmission properties than polymer optical waveguides, but a vitrifaction process (at above 1200° C.) carried out after depositing oxide fine particles, and etching treatment are required, and hence strict manufacturing conditions over a long time are required in the manufacture. On the other hand, with regard to polymer optical waveguides, a thin film can easily be formed using a spin coating method, a dip coating method or the like, and moreover manufacture can be carried out through a low-temperature process using reactive ion etching (RIE) or photolithography. In particular, optical waveguides formed by using photolithography can be manufactured in a short time, and hence have the advantage of being able to be formed more easily and at lower cost than quartz optical waveguides.
As materials used for polymer optical waveguides, polysiloxanes having high thermal resistance have been proposed, and by introducing phenyl groups, methyl groups, ethyl groups or the like into the polymer, control of the refractive index and improvement of cracking resistance have been accomplished. Moreover, art, in which a polysiloxane material known as being heat-curable is made to be radiation-curable by introducing radiosensitive groups therein, has also be reported (see Japanese Patent Application Laid-open No. 2000-66051 and Japanese Patent Application Laid-open No. 6-109936). However, the C—H bonds contained in alkyl groups such as methyl groups exhibit the second harmonic arises in the 1.55 μm wavelength band, causing an increase in the loss in this band. To avoid this, polysiloxanes, in which alkyl groups are not used but all are substituted with phenyl groups, have been reported, but the hardness of a thin film increases, and there is a tendency for a thin film to cause cracks during manufacture. It has been hard to attain both loss reduction and cracking prevention.
On the other hand, an approach, in which C—H bonds contained in the polysiloxane are substituted with C-D bonds or C—F bonds, has been reported, but because this polysiloxane is of a heat-curable type, self core formation using a method such as photolithography cannot be carried out, and hence core formation using a method such as etching has been required (see Japanese Patent Application Laid-open No. 4-157402 and Japanese Patent Application Laid-open No. 2000-230052).
Furthermore, in the art of substituting C—H bonds with C-D bonds or C—F bonds, there have been problems such as the third harmonic of the C-D bonds again arising in the 1.55 μm wavelength band resulting in difficulty in reducing the loss, or peeling away occurring at the core portion/clad layer or clad layer/substrate interface upon the introduction of C—F groups.